Phycocolloids are natural gums produced by and extracted from marine algae. The three principal types of commercially valuable phycocolloids are agar, carrageenan and alginate. Agar and carrageenan are polysaccharides produced in the cell walls of certain red algae or Rhodophyta, while alginate is a cell wall component of certain brown algae or Phaeophyta. Together, these three groups of phycocolloids form the basis for a seaweed processing industry worth several hundred million dollars annually.
Traditionally, the raw materials for agar, carrageenan and alginate have come from harvests of wild populations of marine algae. In recent years, certain types of carrageenan and alginate-producing seaweeds have been cultivated in the Pacific basin, the raw material for carrageenan being produced in the Philippines and Indonesia, and that for alginate in China. However, due to overharvesting of wild populations and increasing demand for products, current supplies of high-quality agar and certain types of carrageenan are insufficient to meet the seaweed processing industry's needs. There is therefore great interest in developing methods for cultivating agar seaweeds, and in expanding cultivation of certain carrageenan seaweeds. In addition, the industry desires new varieties of carrageenan and agar, which are not apparently found in wild plants.
To meet these present and future needs of the seaweed processing and phycocolloid industry, it is necessary to develop genetically improved strains of seaweed which would provide such new products as well as advantageous cultivation properties.
In the past, most seaweed strain improvement efforts have been restricted to the use of classical plant breeding techniques such as strain selection, mutagenesis, and sexual hybridization. Such efforts have produced very few new and commercially valuable strains of seaweeds.
To date, no new and valuable strains of agar- or carrageenan-producing red algae have been produced in the laboratory, primarily because efficient and effective means for producing such strains have not been available. The plant strains currently used in carrageenan cultivation were developed by application of tedious traditional strain selection procedures.
An important traditional method for development of new and improved strains of land plant crop species is sexual hybridization. This has generally involved sexual crossing of closely-related but often different species to produce a hybrid plant that shows hybrid vigor or heterosis. Common agricultural examples include hybrid corn and wheat.
Compared to its important role in land plant crop improvement, application of sexual hybridization to seaweed strain improvement has been extremely limited, especially in red algae. This is due in part to difficulties in obtaining the necessary plants of both sexes of both species. For example, in some carrageenophytes such as Eucheuma, the principal carrageenophyte cultivated, male plants are rare, and in some species, unknown. Furthermore, red algae also appear to lack interspecific interfertility.
Both intraspecific and interspecific crosses in carrageenan-producing red algae have so far failed to yield viable hybrids. For example, efforts to cross different populations of the same species in Chondrus (Chen and Taylor, Botanica Mar., 23, 441-448 (1980)), and in Gigartina (Guiry and West, J. Phycol., 19, 474-494 (1983)), have been unsuccessful in producing fertile hybrid progeny.
Attempts to cross several different species of Gracilaria, the major source of raw material for agar, have also failed. In each case, populations of different Gracilaria species appeared to be incompatible and could not be sexually crossed (e.g., Bird and McLachlan, Botanica Mar., 25, 557-562 (1982); Plastino and De Oliveira, Br. Phycol. J., 23, 267-271 (1988)). Thus, it appears that interspecific sexual hybridization is extremely limited or impossible in the red algae, presumably because of strict incompatibility barriers between species.
Similar incompatibility barriers apparently do not exist in the brown algae, as there are several reports of sexual hybridization between different species. However, in all cases these hybrids have proven to be non-viable or unable to produce progeny. Thus, it appears that sexual hybridization is not a practical method for strain improvement in brown or red algae. New strain improvement techniques are needed for such algae.
Protoplast fusion techniques have been applied to land plants to produce somatic hybrids. These techniques do not require sexual hybridization and so can produce hybrids between two plants of the same or different species regardless of their sexual compatability.
Protoplast fusion has been attempted in several marine algae, but has thus far failed to produce somatic hybrids. A major cause for this failure has been the inability of researchers to regenerate whole plants from the fusion products. Saga, et al, Beihefte zur Nova Hedwigia, 83, 37-43 (1986), have reported an attempt to fuse protoplasts from a green alga, Enteromorpha, with those from Porphyra, a red alga. A heterokaryon or fusion product was apparently formed, but the authors were unable to regenerate whole plants. Protoplast regeneration in general has been accomplished in only a few genera of non-commercial, non-phycocolloid-producing seaweeds to date. Similar efforts to regenerate plants from protoplasts of commercial phycocolloid-producing seaweeds have failed. For example, Cheney, et al., J. Phycol., 22, 238 (1986), describe methods for producing protoplasts in the agarophyte Gracilaria, but were unable to regenerate whole plants from these protoplasts. Likewise, it has not been possible to regenerate protoplasts from carrageenan-producing red seaweeds, or from alginate-producing brown seaweeds into whole plants. Kloareg, et al., Plant Sci., in press, 1989) report isolation of large numbers of protoplasts from the brown alga Macrocystis, but attempts to regenerate whole plants from the protoplasts were unsuccessful.
Successful protoplast fusion and regeneration have been reported in only one genus of seaweed to date, Porphyra, a red seaweed which is known to have excellent regenerative capabilities. Fujita and Mijita, Jap. J. Phycol., 35, 201 (1987), reported regeneration of whole plants from protoplast fusion products of Porphyra, but the resultant plants were chimeras, not true genomic hybrids. As the DNA of the parental plants remains segregated in a chimeric plant, such plants are not the same as hybrids and do not provide the benefits of a hybrid. Thus, chimeras have little or no commercial value for the production of phycocolloids.
Thus, efforts to develop improved strains of phycocolloid-producing marine algae have been frustrated by the inability to apply to red and brown seaweeds currently-used techniques which are applicable to land plants. Sexual hybridization fails because of incompatibility barriers. Somatic hybridization through protoplast fusion fails because protoplasts either cannot be regenerated to whole plants or do not produce true hybrids. In addition, it should be noted that recombinant DNA gene transformation methods (e.g., direct DNA uptake, microinjection) cannot be applied to marine algae because such methods are normally used in conjunction with protoplasts, and protoplasts in phycocolloid-producing marine algae cannot currently be regenerated to whole plants. New and improved methods which allow for successful protoplast fusion and regeneration as well as application of recombinant DNA gene transformation techniques are clearly needed if improved strains of phycocolloid-producing marine algae are to be produced.